This invention relates to zinc sulfide-based electroluminescent phosphors which are used primarily in so-called thick-film electroluminescent (EL) lamps. More specifically, the invention relates to a method of making small particle size electroluminescent phosphors and to a powder containing small phosphor particles.
A cross-sectional illustration of a conventional thick-film EL lamp is shown in FIG. 1. The lamp 2 has two dielectric layers 20 and 22. A first conductive material 4, such as aluminum or graphite, coated on a plastic film 12b forms a first electrode of the lamp 2; while a thin layer of a transparent conductive material 6, such as indium tin oxide, coated on a second plastic film 12a forms a second electrode. Sandwiched between the two conductive electrodes 4 and 6 are two layers 20 and 22 of dielectric material 14 which can be, for example, cyanoethyl cellulose or cyanoethyl starch. Adjacent to the first electrode 4 is a layer of dielectric material 14 in which are embedded particles of a ferroelectric material 10, preferably barium titanate. Adjacent to the second electrode 6 is a layer of dielectric material 14 in which are embedded particles of an electroluminescent phosphor 8. The phosphors available for thick-film EL lamps are primarily comprised of zinc sulfide that has been doped with various activators, e.g., Cu, Au, Ag, Mn, Br, I, and Cl. Examples of these phosphors are described in U.S. Pat. Nos. 5,009,808, 5,702,643, 6,090,311, and 5,643,496. Typically, the individual particles of the EL phosphors are encapsulated with an inorganic coating in order improve their resistance to moisture-induced degradation. Examples of such coatings are described in U.S. Pat. Nos. 5,220,243, 5,244,750, 6,309,700, and 6,064,150.
Copper-activated zinc sulfide electroluminescent (EL) phosphors (ZnS:Cu) are well known. Such phosphors are typically co-activated with Cl and/or Mn. Examples of these phosphors and their methods of manufacture are described in U.S. Pat. Nos. 4,859,361, 5,702,643, and 6,248,261. Some of the applications using electroluminescent phosphors require that the phosphors have small particles sizes. For example, EL phosphor paints used for decorating purposes require small particle size EL phosphors in order to make a stable paint suspension. Manufacturers of EL lamps are also interested in small size EL phosphor powders because they can produce more homogeneous EL phosphor layers and also save material costs since less phosphor is needed for a similarly sized lamp.
U.S. Pat. No. 5,643,496 describes how to make EL phosphors with a particle size under 23 μm. U.S. Pat. No. 6,702,958 describes how to make EL phosphors with particle sizes between 10 and 20 μm, with added emphasis on particle size distribution. However, solid state synthesis methods of making small particle size EL phosphors have not been well established. To obtain a material with a particle size under 15 μm, for example, the previously disclosed methods may generate large, wasteful, coarse portions which are subsequently discarded because the as-synthesized particle sizes were normally over 20 μm.